Aging and the syndrome of obstructive sleep apnea, which is characterized by chronic intermittent hypoxia (CIH), are commonly associated with increased incidence and severity of hypertension, orthostatic intolerance, and cardiovascular diseases. However, our understanding of the neural mechanisms underlying these dysfunctions is impeded by a lack of structural information on autonomic nerve terminals and the circuitry within the cardiac tissues. Vagal projections to the heart originate from a sensory ganglion, i.e., the nodose ganglion, and the motor neuron pools in the brainstem, i.e., the nucleus ambiguus (NA) and the dorsal motor nucleus of the vagus (DmnX). The overall goal of the present application is to determine the functional deficits of the vagal control of the heart induced by aging, CIH, or both, and to identify the damage to the cardiac neural circuitry, specifically to the vagal axonal projections to the heart. Vagal control of particular cardiac functions will be measured in young (3months), middle age (12months) and aged (24 months) Fischer 344 rats. The vagal cardiac axons and terminals, and glutamatergic transmission within the NA and DmnX will be examined qualitatively and quantitatively using a battery of techniques that will include anterograde neural tracing, stereological counting, confocal microscopy, Neurolucida digitization, and dual immunohistochemistry. These anatomical findings will be assessed in conjunction with physiological responses to enhance our understanding of structure-function relationships. Aim 1 will assess aging-associated attenuation of baroreflex and vagal control of the heart, and the associated structural changes of vagal projections to the heart and aortic arch. Aim 2 will evaluate CIH-induced reduction of baroreflex sensitivity and vagal controls and the parallel vagal cardiac axon degeneration. Aim2 will also determine whether aging and CIH interact to induce more severe functional and anatomical damage to the vagal cardiac axons. Aim 3 will study changes in glutamatergic transmission within the caudal brainstem complex (NTS, NA, DmnX) during aging and following CIH. Collectively, the proposed experiments will advance our knowledge of brain-heart interactions and provide unique insights into the remodeling of vagal outflow to cardiac tissues during aging and following CIH.